A re-evaluation of sperm ultrastructure in the emu, Dromaius novaehollandiae

Abstract:

Existing reports on sperm structure in the emu do not adequately illustrate or describe all
the salient ultrastructural features necessary for a meaningful comparison of normal and
abnormal sperm in this species. As sperm morphology forms an important parameter in
determining semen quality, and in view of the proposed role of artificial insemination in
the farming of ratites, this article re-evaluates and complements the existing data on the
topic, provides a fully illustrated description of emu sperm ultrastructure, and documents
some unreported morphologic features. Conventional transmission and scanning electron
microscopy and high resolution scanning electron microscopy were used to describe the
ultrastructure of sperm harvested from the distal deferent duct of sexually mature birds
slaughtered during the breeding season. In addition to broadly confirming the basic ultrastructural
characteristics previously described for emu sperm, this study revealed a
number of unreported morphologic features. These included distinct differences in surface
properties between the acrosome and nucleus, the presence of a thread-like appendage
near the base of the nucleus, variable positioning of the annulus relative to structures
located at the midpiece–principal piece junction and regional differentiation of the principal
piece. Although the emu displayed similar basic morphologic features to sperm of
other ratites and the tinamou, marked structural peculiarities were obvious, notably the
lack of an endonuclear canal and a perforatorium and the presence of significantly more
mitochondria in the midpiece coupled with an absence of intermitochondrial cement.
Although the broad morphologic features of emu sperm would appear to add credence to
the general view that the ratites, together with the tinamous, form a monophyletic group
at the base of the avian phylogenetic tree, it is also clear that emu sperm are distinctly
different from those of the ostrich, rhea, and tinamou which together share morphologic
affinities. This observation may lend some support to the alternate view that the Australasian
ratites represent a separate clade that developed independently from flightless
ancestors.